1. Field of the Invention
The present invention relates generally to the mounting and connecting of devices and, more particularly, to the mounting and connecting of microelectronic units, such as semiconductor chips, on printed wiring boards.
2. Description of the Prior Art
Early methods of mounting and connecting semiconductor chips to printed wire boards frequently resulted in unreliable connections. Specifically, the early methods provided an electrical connection between a semiconductor chip and a printed wire board that consisted of a solder joint. Although suitable for normal environments, such as desk top use, this type of connection proved unreliable in external environments that subjected the board and chip to vibrations and temperature variations. The vibrations frequently caused fatigue failures in the solder joints. Temperature variations caused connection failures due to the difference in the thermal coefficients of expansion (xe2x80x9cTCExe2x80x9d) for the semiconductor chips and the printed wiring board (xe2x80x9cPWBxe2x80x9d). A material""s TCE is the rate at which the material expands or contracts in relation to its temperature. Printed wiring boards, for example, frequently have a TCE that is about three times greater than the TCE for semiconductor chips. This difference in the TCE between the semiconductor chip and PWB frequently caused solder joint strain on early chip mounted boards often interrupting the electrical connections between the chip and the printed wiring board.
To solve this problem, manufacturers developed improved methods of connecting semiconductor chips to printed wiring boards. For example, manufacturers developed peripheral grid array (xe2x80x9cPGAxe2x80x9d) chips configured to have leads arranged about the chip""s periphery. The PGA chip design initially incorporated S-shape leads to compensate for the differing TCE between the PGA chip and the PWB. See U.S. Pat. Nos. 4,827,611; 5,294,039; and 5,317,479.
However, the drive to miniaturize semiconductor chip and PWB assemblies soon led to the development of C-shaped leads, as the S-shaped lead left too much space between the surface of the PWB and the semiconductor chip. The C-shaped lead reduced the spacing between the surface of the chip and the PWB and thus provided a chip with a lower profile, when mounted, than a chip equipped with S-shaped leads. Reducing the lead length also enhanced performance by correspondingly reducing lead inductance. When used in external environments that subjected the mounted assembly to vibration and wide temperature variations, the C-shape retained the lead""s ability to compensate for the differing TCE of the chip and printed wiring board.
Prior to the advent of area grid array (xe2x80x9cAGAxe2x80x9d) semiconductor chips, the C-shaped lead and the S-shaped lead proved adequate in dealing with the problem of differing TCE between semiconductor chips and printed wiring boards. With AGA chips, however, the conductive connecting surface pads of the chip are arranged in a matrix. Each connecting surface pad in the matrix is electrically coupled to a similar conductive pad located within a reciprocal corresponding matrix on the PWB. The AGA chip is connected typically to the PWB via solder joints, each of which is individually formed into a spherical shape. AGA chips employing the typical solder ball joints are sometimes referred to as ball grid arrays (xe2x80x9cCBGAxe2x80x9d).
FIGS. 1 and 2 illustrate a prior art BGA. FIG. 1 illustrates an AGA chip 50 connected to a printed wiring board 70 using solder balls 90 and solder joints 55 and 77. Solder balls 90 are typically made from conventional solder such as Sn63:Pb37, i.e., 63 weight percent tin and 37 weight percent lead, or Sn10:Pb90, i.e., 10 weight percent tin and 90 weight percent, or an equivalent alloy. Typically, plastic BGA (xe2x80x9cPBGAxe2x80x9d) packages use the more common Sn63:Pb37 solder balls and ceramic BGA (xe2x80x9cCBGAxe2x80x9d) packages use Sn10:Pb90 solder balls. However, like the original semiconductor solder joints, solder ball joints are not very reliable when AGA chip 50 and PWB 70 are subjected to temperature variations and/or mechanical vibration.
Moreover, once AGA chip 50 is mounted on PWB 70, accessing a connection point between a single conductive pad on AGA chip 50 and a reciprocal conductive pad on PWB 70 is difficult. When a solder ball joint fails, the entire AGA chip 50 must be removed from PWB 70 in order to effect repairs. While BGA packages have provided space reduction between the chip and PWB, the reliability problems associated with solder joints between semiconductor chips and printed wiring boards have continued.
One attempted solution includes the use of solder columns instead of solder balls. The solder columns are typically made of Sn10:Pb90 solder alloy. Although solder columns enhance compliancy, the columns bend easily and often experience problems as a result of handling during production. Solder columns also fail to provide improved strength or reliability over solder balls. In addition, the high lead content of this solder alloy is highly undesirable due to environmental concerns over the introduction of additional lead into the environment.
Attempts have been made to use a conductive lead to connect an AGA chip to a PWB. For example, U.S. Pat. No. 5,455,390 discloses a method for placing a plurality of conductive connecting leads between the conductive surface pads of the AGA chip and the connecting surface pads of the PWB. However, this method still results in connection failures due to the less reliable type of material, e.g., gold, used to make the conductive connecting leads.
U.S. Pat. No. 6,000,126 issued to the present inventor discloses an improved method of interconnecting an AGA chip to a PWB. This method includes orienting a first side of a matrix of a plurality of conductive leads, secured relative to one another in parallel by an insulating carrier, so that the first ends of the matrix are aligned with a reciprocating matrix of conductive surface pads on an AGA chip. The leads are electrically connected to the connecting surfaces of the AGA chip. The second side of the matrix of leads is oriented so that the second ends are aligned with a reciprocating matrix of connecting surface pads on a PWB. The leads of the second side of the lead matrix are electrically connected to the connecting surface pads of the PWB thereby establishing an electrical connection between the AGA chip and the PWB.
While the method described in U.S. Pat. No. 6,000,126 offers substantial advantages over the prior art, implementation remains relatively expensive due to manufacturing issues and the availability of materials. Also, minimum lead length required for automatic insertion into an insulating carrier may be too long to accommodate some high density, high performance electronic requirements. In addition, most copper alloys used for drawing the leads for the automatic insertion process have relatively low thermal conductivity and thus exacerbate thermal management problems associated with high power components.
Thus, it would be desirable to have an inexpensive means to interconnect an AGA chip with a PWB that maintains high operational reliability across a vast range of operating temperatures and mechanical stresses.
The present invention comprises a method of utilizing inexpensively manufactured, electrically conductive and mechanically compliant disks to interconnect an area grid array chip to a printed wiring board. An automated punch press can be used to stamp disks from a thin sheet of conductive material. The conductivity of the material should be at least 270 kilo mho-cm. To increase solderability and protect the disk surface from oxidation, the disks can be plated with tin or an equivalent material. Conductive pads are located on the surface of an AGA chip in a specific orientation. Each disk is positioned tangent to a corresponding conductive pad. The preferred method of connecting the conductive disks to an AGA chip includes aligning a plurality of conductive disks, secured relative to one another by a rigid nest, in an orientation that corresponds to the specific orientation of the pads on the AGA chip surface. One edge of each disk is then electrically connected and mechanically secured to the connecting surface of the AGA chip. Next, the securing nest is removed. The opposite edge of each conductive disk, that is the point 180xc2x0 from the point tangent to the AGA chip, is then in position to align with a corresponding conductive surface pad on a PWB. The opposite edge of each conductive disk is then electrically connected and mechanically secured to its corresponding connecting pad located on the surface of the PWB, thereby establishing a compliant electrical connection between the AGA chip and the PWB.
In a preferred embodiment, the thin conductive disks of the present invention have an aspect ratio, i.e., the ratio of disk diameter to disk thickness, of about 3.5 to about 10. Thin conductive disks provide more mechanical compliancy than the high temperature solder balls or conductive wire leads well known in the art, and thus can better accommodate TCE mismatch between ceramic components, solder joints, and printed wiring boards. This capability enables electronic assemblies incorporating the disks and method of the present invention to operate reliably over a wide temperature range. In a preferred embodiment, copper disks provide the additional thermal and electrical conductivity required by ever more robust components that consume ever increasing amounts of power. However, conductive disks formed from any material with tensile and shear strength comparable to that of copper, silver, and aluminum and having a conductivity of at least 270 kilo mho-cm will work.
Using an automated hole punching press, disks can be fabricated with tight mechanical tolerances, typically on the order of+/xe2x88x920.0005 inches or less. By comparison, solder balls have diameter tolerances that typically vary from+/xe2x88x920.0007 inch to+/xe2x88x920.003 inch. Tighter mechanical tolerances improve the coplanarity of components, which in turn improves solder joint uniformity, thereby further enhancing reliability. The disks are also lighter in weight than lead solder balls of equal diameter.
In addition to enhancing electrical and thermal conductivity and reliability in both favorable and unfavorable external environments, the conductive disks of the present invention offer an inexpensive method of replacing the conductive solder balls of a BGA with lead-free, more environmentally friendly metals. Disk grid arrays thus provide an economically feasible way to advance the lead-free initiatives advocated by many governments around the globe. The present invention can use xe2x80x9clead-freexe2x80x9d solder and can be easily applied to new ceramic chip scale packages (xe2x80x9cCCSPxe2x80x9d) and plastic grid arrays (xe2x80x9cPGAxe2x80x9d).
In alternative embodiments of the present invention, the disks may be attached by solder, conductive adhesives, or socket or compression fittings.